From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 898FAFA3728 for ; Wed, 16 Oct 2019 16:48:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 68C862168B for ; Wed, 16 Oct 2019 16:48:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2406429AbfJPQsm (ORCPT ); Wed, 16 Oct 2019 12:48:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57100 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2404582AbfJPQsl (ORCPT ); Wed, 16 Oct 2019 12:48:41 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 75A6018C890F; Wed, 16 Oct 2019 16:48:41 +0000 (UTC) Received: from [10.36.116.53] (ovpn-116-53.ams2.redhat.com [10.36.116.53]) by smtp.corp.redhat.com (Postfix) with ESMTP id 699C56CE5A; Wed, 16 Oct 2019 16:48:38 +0000 (UTC) Subject: Re: [PATCH V2] mm/page_alloc: Add alloc_contig_pages() To: Anshuman Khandual , Michal Hocko Cc: linux-mm@kvack.org, Mike Kravetz , Andrew Morton , Vlastimil Babka , David Rientjes , Andrea Arcangeli , Oscar Salvador , Mel Gorman , Mike Rapoport , Dan Williams , Pavel Tatashin , Matthew Wilcox , linux-kernel@vger.kernel.org References: <1571223765-10662-1-git-send-email-anshuman.khandual@arm.com> <40b8375c-5291-b477-1519-fd7fa799a67d@redhat.com> <20191016115119.GA317@dhcp22.suse.cz> <20191016124149.GB317@dhcp22.suse.cz> <97cadd99-d05e-3174-6532-fe18f0301ba7@arm.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Wed, 16 Oct 2019 18:48:37 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <97cadd99-d05e-3174-6532-fe18f0301ba7@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.70]); Wed, 16 Oct 2019 16:48:41 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16.10.19 17:31, Anshuman Khandual wrote: > > > On 10/16/2019 06:11 PM, Michal Hocko wrote: >> On Wed 16-10-19 14:29:05, David Hildenbrand wrote: >>> On 16.10.19 13:51, Michal Hocko wrote: >>>> On Wed 16-10-19 16:43:57, Anshuman Khandual wrote: >>>>> >>>>> >>>>> On 10/16/2019 04:39 PM, David Hildenbrand wrote: >>>> [...] >>>>>> Just to make sure, you ignored my comment regarding alignment >>>>>> although I explicitly mentioned it a second time? Thanks. >>>>> >>>>> I had asked Michal explicitly what to be included for the respin. Anyways >>>>> seems like the previous thread is active again. I am happy to incorporate >>>>> anything new getting agreed on there. >>>> >>>> Your patch is using the same alignment as the original code would do. If >>>> an explicit alignement is needed then this can be added on top, right? >>>> >>> >>> Again, the "issue" I see here is that we could now pass in numbers that are >>> not a power of two. For gigantic pages it was clear that we always have a >>> number of two. The alignment does not make any sense otherwise. > > ALIGN() does expect nr_pages two be power of two otherwise the mask > value might not be correct, affecting start pfn value for a zone. > > #define ALIGN(x, a) __ALIGN_KERNEL((x), (a)) > #define __ALIGN_KERNEL(x, a) __ALIGN_KERNEL_MASK(x, (typeof(x))(a) - 1) > #define __ALIGN_KERNEL_MASK(x, mask) (((x) + (mask)) & ~(mask)) > >>> >>> What I'm asking for is >>> >>> a) Document "The resulting PFN is aligned to nr_pages" and "nr_pages should >>> be a power of two". >> >> OK, this makes sense. > Sure, will add this to the alloc_contig_pages() helper description and > in the commit message as well. As long as it is documented that implicit alignment will happen, fine with me. The thing about !is_power_of2() is that we usually don't need an alignment there (or instead an explicit one). And as I mentioned, the current function might fail easily to allocate a suitable range due to the way the search works (== check aligned blocks only). The search really only provides reliable results when size==alignment and it's a power of two IMHO. Not documenting that is in my opinion misleading - somebody who wants !is_power_of2() and has no alignment requirements should probably rework the function first. So with some documentation regarding that Acked-by: David Hildenbrand -- Thanks, David / dhildenb